An infrared detector detects the intensity of infrared radiation by measuring a change in the temperature of an infrared detecting device upon which the infrared radiation impinges. To prevent the radiation energy captured by the device from dissipating to outside, the device is kept in a vacuum environment. To produce such an infrared detector, it is known to place a device in a recess of a casing main body and secure a window member onto an opening of the recess by brazing in a vacuum chamber (see Japanese patent laid open publication No. 2003-139616). Brazing material is deposited on a part of the casing main body surrounding the opening, and the casing main body and window member are heated by corresponding heaters provided in an upper and lower parts of the vacuum chamber to turn the brazing material into molten state. The two parts are then pushed against each other, and are allowed to cool off by turning off the heaters until the brazing material solidifies and securely joins the two parts.
In particular, the mentioned patent publication discloses the use of a flange having a metallized surface to ensure a mechanically strong and air tight bonding between the casing main body and window member. More specifically, a metallized layer formed on each of the bonding surfaces of the casing and the corresponding bonding surface of the window member, and is processed so as to have a favorable affinity with molten brazing material. The inner edge of the flange is chamfered. Thereby, a fillet of brazing material is formed along the inner edge of the flange to achieve a reliable bonding.
The method for making such a package typically includes the following steps. First of all, as illustrated in FIG. 10a, a ceramic casing 94 defining a cylindrical cavity 93 is prepared. An infrared detecting device 92 is mounted on the bottom of the cavity 93. A casing annular metallized layer 96 is formed on an annular shoulder surface defined along the part of the casing surrounding the opening of the cavity 93, and a prescribed amount of molten brazing material 98 such as In—Sn and Sn—Ag alloys having a melting point below 250° C. is deposited on the casing annular metallized layer 96 in a vacuum environment. The deposited brazing material 98 is allowed to cool and solidify. Because the metallized layer 96 has a high affinity with the molten brazing material 98, the molten brazing material 98 extends over the entire surface of the metallized layer 96, and assumes a somewhat upwardly bulging shape owing to the surface tension thereof.
This assembly 91 is then taken out of the vacuum environment and the deposited brazing material 98 is suitable machined so as to remove excess part and reveal a clean metallic surface free from oxidized film as illustrated in FIG. 10b. This assembly 91 along with a corresponding window member 95 is then set in a vacuum chamber to perform a baking process. As shown in FIG. 10c, by using a suitable jig, the window member 95 is placed above the ceramic casing 94 and is pressed against the ceramic casing 94 while heating the assembly. Thereby, the brazing material 98 joins the window member 95 with the ceramic casing 94 in an air tight and mechanically secure manner.
However, according to such a conventional method for making an infrared detector package, the loss of the brazing material is significant owing to the need for shaping the brazing material by machining. Also, the metal surface revealed by the machining may not remain clean until the baking or brazing process, and the segregation, formation of dross and other forms of soiling of the metal surface that could cause improper brazing results may occur. It has been proposed to scrub the window member while the brazing material is in molten state, but it is difficult to eliminate the possibility of improper brazing or imperfect sealing. As a result, a significant amount of efforts were required to totally eliminate defects in the products, and the manufacturing cost was undesirably high.